The invention relates to subcarrier spacing in mobile communication systems that employ Orthogonal Frequency Division Multiplexing (OFDM), and more particularly to techniques that enable a user equipment operating in such a system to detect which of a number of possible subcarrier spacings are presently being used for communications.
In the forthcoming evolution of the mobile cellular standards like the Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like OFDM are likely to occur. Furthermore, in order to have a smooth migration from the existing cellular systems to the new high capacity high data rate system in existing radio spectrum, the new system has to be able to operate in a dynamically changeable bandwidth. A proposal for such a new flexible cellular system is 3G Long Term Evolution (3G LTE, where “3G” stands for “Third Generation”) that can be seen as an evolution of the 3G WCDMA standard. OFDM will be used in this system in a technique called OFDM Multiple access (“OFDMA”) to enable multiple users to share access to the radio spectrum in the downlink. The system will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be supported on the largest bandwidth.
Another important aspect of LTE is efficient support for broadcast and Multimedia Broadcast/Multicast Service (“MBMS”). In LTE, so called “Single Frequency Network” (SFN) operation is foreseen in which base stations are synchronized. Here, MBMS content is transmitted from several base stations using the same physical resources. The signals from these multiple transmissions are automatically “combined in the air”, so no additional receiver resources are required for this purpose. In order for this “over the air combining” to work, all participating base stations must be synchronized—both in the frequency domain and in the time-domain—down to the extent of a fraction of the length of a cyclic prefix. In order to ease time synchronization requirements a long cyclic prefix is advantageous. However, increasing the cyclic prefix without increasing the OFDM symbol duration increases overhead and is thus not attractive. One possible solution is to use a smaller subcarrier spacing (and corresponding bandwidth), thus increasing the OFDM symbol duration (the OFDM symbol duration is inversely proportional to the subcarrier spacing). For example, halving the subcarrier spacing results in OFDM symbols that are twice as long, thereby enabling a cyclic prefix that is twice as long. In this manner, the amount of overhead is maintained constant. Therefore, in addition to support for 15 kHz subcarrier spacing, LTE also supports the use of a 7.5 kHz subcarrier spacing for SFN operation.
The physical layer of a 3G LTE system includes a generic radio frame having a duration of 10 ms. FIG. 1 illustrates one such frame 100. Each frame has 20 slots (numbered 0 through 19), each slot having a duration of 0.5 ms. A sub-frame is made up of two adjacent slots, and therefore has a duration of 1 ms.
One important aspect of LTE is the mobility function. Hence, synchronization symbols and cell search procedures are of major importance in order for the User Equipment (UE) to detect and synchronize with other cells. To facilitate cell search and synchronization procedures, defined signals include Primary and Secondary Synchronization Signals (P-SyS and S-SyS, respectively), which are transmitted on a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH), respectively. The P-SySs and S-SySs are each broadcast twice per frame: once in sub-frame 0, and again in sub-frame 5, as shown in FIG. 1.
The UE must detect, as soon as possible, whether it is connecting to a 7.5 kHz/subcarrier cell or a 15 kHz/subcarrier cell, since subsequent procedures may be different for the two cases. One possibility, of course, is to have two completely different synchronization signal designs, each uniquely associated with one of the subcarrier spacing sizes. However, here the UE would be required to have both synchronization signal designs implemented, in which case it would either have to run search algorithms for both synchronization signals in parallel—thus increasing complexity—or sequentially—thus increasing cell search time.
It is therefore desired to have a technique that will enable a UE to detect what the subcarrier spacing is without having to have two different synchronization signal designs.